Top Tip 8 - Cardiology - Atropine Response Test

An atropine test is one of the tests we use at SARC in the cardiorespiratory department to fully diagnose the cause of bradycardia. It is a straightforward test and can be performed in your own practice. All you need is a functional ECG, atropine and the following guidelines.

What is the atropine response test?

The atropine response test is the diagnostic administration of atropine to differentiate vagally-mediated and non-vagal bradyarrhythmias. This is very important in cases of persistent bradycardia as non-vagal bradyarrhythmias, if resulting in clinical signs, usually require pacemaker implantation, where as vagally mediated bradycardias may be of no clinical significance or require treatment for non-cardiac disease.

How do I perform an atropine response test?

There are several ways of performing the atropine response but the author has had the most consistent results with the methods outlined below:

Method 1

1. Record the ECG at baseline.
2. Administer 0.04mg/kg atropine IV
3. Wait 15 minutes
4. Record the ECG for at least 2 minutes (use a slow paper speed).
5. If the response is incomplete, repeat steps 2-4.

Anticipated response: persistent sinus tachycardia at >140bpm is expected in most dogs with vagally-mediated bradycardia.

Method 2

1. Record the ECG at baseline.
2. Administer 0.04mg/kg atropine SQ
3. Wait 30 minutes
4. Record the ECG for at least 2 minutes (use a slow paper speed).

Anticipated response: persistent sinus tachycardia at >140bpm is expected in most dogs with vagally-mediated bradycardia.

What sort of conditions can lead to vagally mediated bradyarrhythmias?

Increases in vagal tone can occur secondary to physiological or pathological conditions. Extreme fitness can result in increased vagal tone, resulting in bradycardia. This is usually seen in elite athletes. Some athletic dogs can have sinus bradycardia or other vagally-mediated bradyarrhythmias.

Pathological conditions that increase vagal tone include chronic respiratory diseases (that exaggerate the respiratory phasic variations in vagal tone), primary CNS disorders (brain tumors), gastrointestinal disorders, and some endocrine conditions (Addison's Disease). The exact mechanisms that result in bradycardia with these conditions are complex, and not always well defined.

Can a vagally-mediated bradyarrhythmia be a primary cardiac problem?

Most of the time, the bradyarrhythmias resulting from high vagal tone are of no cardiac clinical consequence.

What does an abnormal response to an atropine response test mean?

There are 2 general causes for suboptimal responses to an atropine response test:

1) The atropine dose was insufficient to abolish vagal tone. This could be due to technical error (i.e., the dose was miscalculated, the dose was administered incorrectly), or the patient may have exceedingly high vagal tone. Repeating the test is recommended if either of these situations is suspected.

2) The sinus node is diseased and cannot respond appropriately or there is atrio-ventricular block. These are pathological causes of a failed atropine response test. In these cases, patients are usually classified as having intrinsic sinoatrial nodal disease, or “Sick Sinus Syndrome” or third degree atrioventricular (3o AV) block. With 3o AV block the QRS complexes do not increase in frequency (and therefore cardiac output does not increase) but the P-wave frequency normally increases to >140bpm. The recommended treatment for both these presentations is usually pacemaker implantation.

If you are a vet who would like further information on atropine tests or any aspect of cardiology, please feel free to contact Dr Richard Woolley at SARC on (03) 9532 5261

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